Evolution here and now

17022011

Here’s a guest post from one of my PhD students, Salvador Herrando-Peréz. Salva is working on theoretical aspects of density feedback mechanisms among different species, and is especially eclectic with his interests in biology. Salva regularly contributes to lay natural history magazines, especially in his native tongue Castellano (Spanish), and he is an active member of the Spanish organisation Bioestudios Saganta, a non-profit national organisation fully devoted to scientific research and its popularisation with a focus on biodiversity conservation.

I’ve asked my students to start contributing to ConservationBytes.com, and Salva is leading the charge.

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Evolution evokes ideas such as fossils, geological eras and time scales of hundreds of thousands to millions of years. Only recently have we started to appreciate that such ‘macro-evolution’ is the result of accumulated changes in the morphology and genes of species from one generation to the next: days for HIV strands, months for a planktonic rotifer, or years for a poplar.

The Britons Peter and Rose Mary Grant published in 2002 a 30-year study on Darwin’s finches from Daphne Major (Galapagos, Ecuador) – a popular study organism since Charles Darwin’s Origin of species (Grant & Grant 2002). In such a short period of time, covering only six generations of these granivorous birds, several extreme droughts altered the type and abundance of seeds, and potentially triggered the evolution of body size, and beak shape and size, up to three times (Figure 1). The two biologists from Princeton reveal that:

evolution is reversible – generations of finches experiencing overall increase in body and beak sizes can lead to future generations with smaller sizes (of course within limits; a finch will never develop the beak of a stork or a hummingbird), and

phenological shifts across generations are unpredictable in so far as they respond to random climatic fluctuations – should droughts of contrasting intensity have occurred in different years over the study period, beaks and bodies might have evolved in other particular fashions.

This is an example of what we know as ‘rapid evolution’, ‘contemporaneous evolution’ or ‘micro-evolution’ (see rationale of terms in Hendry and Kinnison 1997), in time periods on the order of a human life.

Macro- and micro-evolution were overly academic and laboratory topics from their formal distinction around the 1930s by, among others, the Ukrainian geneticist Theodosius Dobzhansky. However, the last two decades have witnessed a boom of field studies quantifying micro-evolutionary shift.

The Australian red-bellied black snake (Pseudechis porphyriacus) builds up resistance to cane toad toxicity in 23 generations and 40 years (Phillips and Shine 2006). One decade and three generations suffice for Canadian red squirrels (Tamiasciurus hudsonicus) to advance the onset of reproduction by nearly one month in response to precocious spring masting associated with global warming (Réale et al. 2003). In less than 50 years, the soapberry bug (Jadera haematoloma) modified beak length several times relative to fruit structure of three plant species gradually introduced in the south of the USA (Carrol and Boyd 1992). Intensive harvesting of the Himalayan snow lotus (Saussurea laniceps) causes dwarfing of this medicinal plant through the last century (Law and Salick 2005). North-western Atlantic cod (Gadus morhua) reached gradually smaller body sizes and earlier maturation towards the collapse of this fishery in the 1980s (Olsen et al. 2004).

Along this fascinating wave of findings, the Grant & Grant hypothesized that body size and beak structure in Darwin’s finches can deliver visual signals relevant to mating, and condition their food (small beaks would have a hard time handling large seeds, and vice versa) and the species with which they have to compete for foraging. Parallel studies have shown that micro-evolutionary changes in the body of these birds can explain over twice more inter-annual variation in population size than competition for resources free of morphological change (Hairston et al. 2005). Therefore, micro-evolution can shape fertility and survival rates, hence demography: how many individuals there are today and how many there will be tomorrow.

The connection with conservation issues is straightforward in what has been coined as ‘evolutionary rescue’ (Kinnison and Hairston 2007). Those species able to micro-evolve at rates higher than the alarming rates of human-made habitat alteration (a formidable trigger for contemporaneous evolution) could theoretically enhance their persistence (again within limits: a hippo would never make it in the streets of a city). Likewise, small and/or fragmented populations can micro-evolve thereby counteracting the army of mechanisms which can otherwise drive them to extinction (e.g., inbreeding depression) – the colonising success of invasive and pest species from minuscule founding populations illustrates such outcome.

However, as for threatened species, the chances of evolutionary rescue have to be gauged case by case. For instance, reintroductions can fail if individuals have previously micro-evolved in their captive environment (Stockwell et al. 2003). Besides, evolution at any temporal scale is strongly determined by stochastic factors such as trait heritability, environmental cues, or the relative fitness for adaptation and persistence of races of individuals departing from the average characteristics of a species.

Salva, I would have thought that the examples you cited were all micro-evolution in that they are reversible changes that do not result in genetic innovations. This is not my field, but I am interested in what micro-evolution would be?

I am not an evolutionary biologist, however I can agree with a definition of evolution as “genetically-based change in the characteristics of populations and species over time” (Kinnison and Hendry 2001). Yet, and unfortunately, definitions of concepts and the semantic of terms is one of the pending issues of ecology, and vary across authors and publications. Thus, any partition of evolutionary change by time intervals might be appropriate at the extremes of the time scale and arbitrary in between. A clear-cut differentiation I have read in the literature is as follows:

‘Rapid evolution’: scale = less than a few centuries (Hendry and Kinnison 1999)
‘Contemporary evolution’: scale = thousands of years (Hendry and Kinnison 1999)

It is also interesting to consider that microevolution can be measured through rates of change (e.g. in Haldanes or Darwins, see Gingerich 1983). Microevolutionary studies can quantify ‘rates of evolution’ if considering change over time within a population (‘alochronic studies’), or ‘rates of divergence’ if considering change between several populations with a common ancestor (‘synchronic studies’) (see Hendry and Kinnison 1999).

See references above, except:
Dobzhansky T. 1937. Genetics and the origin of species. Columbia University Press, New York.
Kinnison MT. 1999 and Hendry AP. 2001 . The pace of modern Life II: from rates of contemporary microevolution to pattern and process. Genetica 112-113: 145-164